Hi there -- i get the feeling this came in with 5.0 but not totally sure.
If you have a project with very many items, and you do the following actions in sequence:
Item: Select all Items
SWS: Restore saved selected items
...it causes Reaper to hang for a long time, with high CPU.
(I know that that sequence is not useful: I have whittled the bug down to this sequence from a useful macro which used to work and now hangs.)
I see this on win 8.1/64 and Win XP/32. Reaper 5.01 and SWS 2.8.0.
If you reduce the number of items, it eventually gets under a rough threshold where it works fine, but adding items past the threshold makes it seem to take geometrically more time. I.e. as you progressively add items, there's a small window where it hangs for a second or two, but the time it takes before the system is responsive again quickly goes through the roof. Presumably there is some recursive traversing happening or something.
Here is the demo project
I was using to trigger it, in case you need something with lots of items in it.
Again, this didn't happen before, regardless of the number of items. My macro is very dear to me, so fingers crossed that this can be fixed. :-)
Let me know if there is any other way I can assist!